procedure TForm1.HideTitlebar; 
var 
  Style: Longint; 
begin 
  if BorderStyle = bsNone then Exit; 
  Style := GetWindowLong(Handle, GWL_STYLE); 
  if (Style and WS_CAPTION) = WS_CAPTION then 
  begin 
    case BorderStyle of 
      bsSingle, 
      bsSizeable: SetWindowLong(Handle, GWL_STYLE, Style and 
          (not (WS_CAPTION)) or WS_BORDER); 
      bsDialog: SetWindowLong(Handle, GWL_STYLE, Style and 
          (not (WS_CAPTION)) or DS_MODALFRAME or WS_DLGFRAME); 
    end; 
    Height := Height - GetSystemMetrics(SM_CYCAPTION); 
    Refresh; 
  end; 
end; 

procedure TForm1.ShowTitlebar; 
var 
  Style: Longint; 
begin 
  if BorderStyle = bsNone then Exit; 
  Style := GetWindowLong(Handle, GWL_STYLE); 
  if (Style and WS_CAPTION) <> WS_CAPTION then 
  begin 
    case BorderStyle of 
      bsSingle, 
      bsSizeable: SetWindowLong(Handle, GWL_STYLE, Style or WS_CAPTION or 
          WS_BORDER); 
      bsDialog: SetWindowLong(Handle, GWL_STYLE, 
          Style or WS_CAPTION or DS_MODALFRAME or WS_DLGFRAME); 
    end; 
    Height := Height + GetSystemMetrics(SM_CYCAPTION); 
    Refresh; 
  end; 
end; 

procedure TForm1.Button1Click(Sender: TObject); 
begin 
  HideTitlebar; 
end; 

procedure TForm1.Button2Click(Sender: TObject); 
begin 
  ShowTitlebar; 
end;